Wednesday, October 31, 2012

Flooding from Hurricane Sandy caused billions of dollars of damage to property. Low-lying coastal areas were devastated. Tunnels and part of the NYC subway system flooded. Barrier islands got hammered, airport runways flooded, and the list goes on and on.

Here are some photos of the flooding along the eastern seaboard:

New Jersey

NYC subway

Atlantic City, NJ

Delaware

Hoboken, NJ

Brooklyn, NY

New York

Rodanthe, North Carolina

Staten Island, NY

New York, flooded airport runway

Ground Zero Site, NYC

Maryland

Subway, Hoboken, NJ

New Jersey

Why was the flooding so bad?

Six factors combined to make flooding pretty much as bad as it could be. They are:

Sea level rise

Full moon and high tide

Hurricane low pressure

Hurricane-force winds and associated storm surge

Low-lying coastal areas

Shallow sloping shoreline

First: Sea Level Rise

A report in National Geographic summarizes observations about sea level rise along the east coast of the United States. It states that sea level rise is occurring nearly twice as fast along the east coast as the global average. You can read more about that by clicking this link:

Sea level has been rising between Cape Hatteras, NC, and Boston, MA at the rate of 2.0-3.8 mm/year between 1950 and 2009. If we go with a middle value of 3 mm/year, then sea level has risen about 7 inches since 1950. That may not sound like a lot, but it becomes significant when you start looking at flood conditions. All indications are that the rate of sea level rise is increasing as global warming progresses.

Second: High Tide

People directly affected by weather and flooding from Hurricane Sandy wouldn't have seen this, but there was a full moon on 10/29/2012. The height of ocean tides are affected by the relative positions of the Earth, Sun, and Moon. High tides are highest and low tides are lowest when the Earth, Sun, and Moon all line up in the same plane. This happens when we have a full moon and a new moon. Unfortunately, it was a full moon on 10/29, the same night Hurricane Sandy came ashore. This means that the tides that night were already higher than normal.

Third: Low Air Pressure

A hurricane is a low pressure system. This means that in the eye of the storm in particular and the whole storm in general has lower air pressure than high pressure systems have. In order to understand this part of the equation you need to imagine the entire height of the atmosphere above your head. It extends upward 100s of miles, but most of the mass of the atmosphere is in the few miles directly overhead.

The weight of the atmosphere directly overhead produces the air pressure we experience. Interestingly, high pressure pushes down on water, causing tides to be lower than they would otherwise be. And, vice versa, low air pressure allows tides to be higher than they would otherwise be. How much of a difference? A change in 1mb (millibar) of air pressure relates to up to 1cm of tidal height when high pressure pushes down on the water surface. When air pressure is low, however, it may allow tides to be a bit higher, but it does not by itself drive tides significantly higher than predicted.

Average sea level air pressure is about 1013mb. The air pressure in the middle of Hurricane Sandy was 946mb when it came ashore. This ties the lowest air pressure for a hurricane making landfall this far north. That last one was in 1938!

This means that air pressure did not mitigate tidal heights.

Fourth: Storm Surge

Storm surge is the biggest factor in coastal flooding associated with hurricanes. The height and effect of storm surge is determined by several factors: storm intensity, tidal height, angle of waves to shorelines, presence of bays and inlets, slope of the shoreline, etc.

Here's what happens. As a hurricane approaches shore the effects of tides are felt first. So the first significant effects are felt as tides rise, often well above normal because of the amount of water being pushed by the storm. Then, waves produced by the storm start coming ashore. These tend to increase in size as time goes on. This is because wave size is determined mainly by two factors: the strength of wind and fetch (the distance wind blows across water).

Waves produced by hurricanes can be huge because both wind velocity and fetch are massive. Hurricane Sandy, for example, was over 1000 miles across. And though windspeed didn't get high enough to reach more than category 1 status, the wind it produced blew over vast expanses of ocean.

So once the tide was in and Sandy came ashore, wave after wave piled up on the shore with no way for the water to get back offshore, so it was pushed farther and farther inland. This is the water that flooded subways, tunnels, airports, etc., etc.

There are some good animations that demonstrate the combined effects of tide and storm surge. You can view the by clicking these links:

Fifth: Low-Lying Areas
The coastal flooding was particularly bad because the NJ, NYC area is low-lying. This means that there was not much there to slow or stop the high storm tide (regular tide + storm surge) that Hurricane Sandy produced.

Sixth: Shallow sloping seafloor and narrow passages between landmasses
This image of the greater NYC area shows that this highly populated area is clustered on islands and land masses separated from each other by narrow waterways. This means that when the storm tide (tide + surge) pushed into these areas, water stacked up and spilled more readily onto land. This had to contribute significantly to the flooding as well.

Wrapping up
So when you combine sea level rise, high tide, low air pressure, storm surge, and local geography with a storm the size of Sandy, that's a recipe for disaster!

Tuesday, October 30, 2012

Hurricane Sandy is probably the biggest piece of weather news we've had all year (at least that people paid much attention to). It is a massive storm that now (10-30-2012) has affected millions of people and caused billions of dollars worth of damage. It's so big that its weather effects are being felt as far inland as Ohio and Indiana.

Hurricane Sandy, 10-28-2012

(Image courtesy of NASA Observatory Earth)

This is NASA satellite photo (above) shows some of Hurricane Sandy, and the video below shows how the entire storm spins as air spirals toward the eye of the storm.

If you've ever wondered why hurricanes move like this, then this is your lucky day. I'll do my best to explain why this happens.

First of all, a hurricane grows out of a tropical depression (tropical low pressure system). A tropical depression is a weather system where sea surface temperatures are high and the air is loaded with moisture due to sea surface evaporation. The resulting warm, moist air is extremely unstable and less dense than the air around it, so it rises into the upper atmosphere (troposphere, actually - the layer of the atmosphere right next to the Earth's surface, 3-10 miles thick).

As warm, moist air continues to rise a low pressure region forms. This means that as air moves into the upper atmosphere it has to be replaced by air from neighboring air masses. You can imagine a low pressure system to act like a valley or depression that neighboring air flows into.

The top image above shows a side view of a low pressure system (at least one way to imagine it). Warm moist air rises, and that air is replaced by air from surrounding areas. The larger and stronger a low pressure system is, the farther away it can pull air in.

The lower image shows a top view of a low pressure cell. Imagine air in the center of the low pressure area moving up toward you and air from nearby areas flowing toward the low pressure area to replace the air that rose and moved into the upper troposphere. Well, those arrows show what air would do if the Earth didn't spin.

Because the Earth rotates and moving air is not physically attached the surface, the Earth rotates under moving air. Resulting physical effects, collectively called the Coriolis Effect, causes the path of air or water currents to deflect to the right in the northern hemisphere, and to the left in the southern hemisphere.

The upper image shows Coriolis Effect on air moving toward the low pressure region. Coriolis Effect deflects the moving air to the right as it moves, in this case approaching the center of the low pressure cell. The blue arrows in the upper figure shows how air would move if the Earth did not rotate, but the peach colored arrows show the movement of air under the influence of the Coriolis Effect.

The map below shows that most of the air moving toward the storm center ends up moving more or less parallel to the eye of the storm. This is why there is usually little air movement at all in the eye, except upward. This deflection to the right occurs at all distances from the eye. The stronger the winds are, and the farther they blow, and the larger the Coriolis Effect. This ends up making an entire hurricane spin in a counterclockwise direction (in the northern hemisphere - it's opposite in the southern hemisphere).

(Image courtesy of NASA)

The low pressure cell at the center of a hurricane is extremely powerful and pulls air in from hundreds of miles away. Wind blowing over these long distances toward the strong low pressure cell at the eye of a hurricane deflects significantly and create a significant spiraling wind pattern.

Tuesday, October 23, 2012

Record sea ice melting in the Arctic Ocean receives a lot of attention from the media - as far as climate change news goes - but you don't hear that much about what is happening in the Antarctic.

First of all, a quick reminder about sea ice in the Arctic Ocean:

Sea ice melt in the Arctic Ocean in 2012 smashed the previous record by 750,000 km2. The map below shows the observed sea ice extent in Sept 2012 (white area) compared to the 1979-2000 average extent (pink line) Wow!

Sea ice extent is defined as the area of the sea with at least 15% sea ice cover. The graph below shows the Arctic sea ice extent for the years 2007-2012 and the 1979-2000 average. Sea ice melt for all individual years shown (2007-2012) have minimum sea ice extents that are significantly (statistically) less than the 1979-2000 average (dark gray line; lighter gray area is + 2 standard deviations around the 1979-2000 average). The bottom line for the Arctic is that it is warming significantly, and much faster than even the fastest climate models developed to date.

Ok, let's take a look at what's happening in the Antarctic:

The map below shows the Antarctic maximum sea ice extent (white area) for 2012 compared to the 1979-2000 average sea ice extent (orange line). 2012 sea ice extent in the Arctic set a new sea ice maximum record. The graph below the map shows the sea ice extent for 2012 compared to the 1979-2000 average.

The graph below shows average sea ice extent for the month of Sept for 1979-2000 and for selected individual years. Interestingly sea ice extent is increasing on average around Antarctica. When we look at data of sea ice cover in recent years in the Antarctic we see that 2006, 2007, 2011, and 2012 all had higher than average sea ice extent maxima but only 2006 and 2012 maximum extents were statistically higher (different) than the 1979-2000 average. With that being said, sea ice extent did exceed the + 2 standard deviation range in 2006 and 2012. So, what is going on in the Antarctic that is leading to increased sea ice cover?

Here's a brief summary of their report together with some additional information to increase clarity:

Temperatures are warming in the Antarctic, just not as fast as in the Arctic (the NSIDC cites references you can refer to if you want more info on this.)

Warming of the Pacific Ocean and ozone depletion over Antarctica combine to strengthen circumpolar winds.

The strongest of these circumpolar winds blows east to west, and Coriolis Effect causes these winds to deflect to the left (north).

The northerly flow of air around most of Antarctica causes sea ice to be pushed farther north than usual, spreading it out and increasing sea ice extent (remember, sea ice extent = 15% ice cover or more)

It would be very interesting to know whether the total amount of sea ice being formed in the Antarctic is increasing or decreasing. All we know right now is how the ice that is being formed is being dispersed.

In summary:

Sea ice extent around Antarctica is increasing, but it is not increasing because it is getting colder. It is increasing because winds blowing toward the north are dispersing sea ice farther away from the Antarctic coastline than usual. Don't forget that sea ice formation in the Antarctic winter is followed by nearly 100% sea ice melt in the Antarctic summer - this is different than in the Arctic where multiple-year sea ice has historically accumulated.

Monday, October 8, 2012

This is reading #2 in a series that I'm developing for my future marine biology students. Please leave a comment below if you find typos or gross inaccuracies. Citations to references showing needed content changes are appreciated. Thanks!

********************************************Brief Geologic History
and Zonation of the Ocean

Objectives:

1.Be able to provide a historical framework for
major events that happened in the ocean.

2.Be able to label all oceans and major seas on a
map of the world.

3.Be able to draw a cross-sectional image of an
ocean basin, and label and define all seafloor features.

4.Be able to describe two different ways volcanic
islands are formed.

5.Be able to list and describe all benthic zones.

6.Be able to list and describe all oceanic zones.

Introduction

The ocean is the largest life-supporting
habitat on the planet.It covers 70.9%
of the Earth’s surface, has an average depth of 3700 meters, and contains over 1.3
billion km3 of living space.In addition, the ocean is home to at least half of all known species,
yet over 95% of it still remains unexplored.

Geologic History of the Ocean

The Earth is 4.5 billion years old.It was so hot at first that water existed
only as water vapor.By 4 bya (billion
years ago) the atmosphere and planet cooled enough that water vapor condensed and
filled ocean basins for the first time.This deluge also produced continental runoff that brought sediment and
salts to the ocean. Ocean salinity
increased until 1.5 bya when it stabilized at concentrations we observe today.We do not completely understand the processes
that maintain ocean salinity at the current stable level.We do know that salt is lost from water by
physical and chemical processes and that salt is added to water by an influx of
material from the continents.

Life evolved in the sea.The first evidence of life appeared about 3.5
billion years ago.The first organisms
were prokaryotes, including the photosynthetic cyanobacteria. It took
cyanobacteria nearly a billion years of ongoing photosynthesis to produce
enough oxygen so that it could start accumulating in the atmosphere and surface
waters of the ocean.Early oxidation of
the atmosphere and ocean occurred 2.5 to 1.8 bya in what we call the great oxidation event (Fig.
2-1).Little evidence of further
increases in ocean or atmospheric oxygen occurred between 1.8 – 0.8 bya and is
called the boring billion.The first eukaryotes appeared 1.5 bya, however,
making that billion years not totally boring.Between 0.85-0.54 bya enough oxygen accumulated in the atmosphere and
ocean that even the deep sea became oxygenated.The Earth also experienced alternating hothouse and ice age conditions during
that time.Oxygen levels continued to
increase, and about 530 million years ago the ocean experienced a massive
proliferation of anatomically complex animal life.This is called the Cambrian Explosion.The oldest
fossils of most modern animal body plans were produced at this time.Oxygen concentrations in the atmosphere and
ocean stabilized to modern levels a few hundred million years ago and have been
the same ever since.

The size and shape of the ocean is
in slow but constant flux.Changes occur
as tectonic forces create new
oceanic crust in some places and drive subduction
in others.These forces push and pull continental
plates around the Earth’s surface at the a few to several cm yr-1. That’s about the same rate your fingernails
grow.Sometimes tectonic forces move the
continents together creating a supercontinent
like Pangea (Fig. 2-2). When a
supercontinent exists the rest of the Earth is covered by one massive
ocean.At other times, like now, the
continents are dispersed around the planet surface, and the ocean is divided into
several smaller basins (Fig. 2-3).

Figure 2-2. Tectonic
movement of contents between 250 mya and today. Pangea is the most recent
supercontinent. (Image: USGS)

Figure 2-3. Boundaries of oceans and major seas of the
modern world. (Image: NOAA)

Seafloor Topography

The seafloor extends from sea level
at the margins of all continents and islands down to ocean trenches more than
10,000 meters below the surface. Figure
2-4 shows a generalized profile of the seafloor and major features associated
with it.These features include the
continental shelf, shelf break, continental slope, continental rise, abyssal
plain, volcanic islands, trenches, seamounts, and mid-oceanic ridges.

The continental shelf is the submerged edge of a continent.Continental shelves extend a few kilometers
to over 1000 kilometers in width, and the outer edge of the shelf is a few
hundred to several hundred meters deep.The continental shelf break marks
the outer edge of the continental shelf.This is where the shallow grade of the continental slope gives way to
the steep grade of the continental slope.The continental slope plunges
down a few thousand meters before it reaches the continental rise.The continental
rise is the transitional area that shifts gradually from the steep grade of
the continental slope to a flat abyssal plain.The abyssal plain may be 3000-6000 meters deep and is a vast, muddy
expanse covering most of the seafloor, though volcanic islands, trenches,
seamounts, and mid-oceanic ridges interrupt it.

Volcanic islands form along trenches
where oceanic crust subducts under another tectonic plate (Fig. 2-5). Islands that form along trenches typically
form an island arc.A couple of examples
of island arcs include the Aleutian Islands and the Marianas Islands (Fig 2-6).Volcanoes form as subduction pushes crust
material and water trapped in the sediment downward.Heat from the mantle superheats the subducted
rock and water, but since that material is now under extreme pressure the water
remains liquid and facilitates the further heating of rock around it. Lower density rock in the subducted crust
becomes semi-pliable and gradually rises toward the surface.When this superheated rock material gets close
enough to the surface, pressure is reduced and the rock can transition into
magma that is released during a volcanic eruption.Ongoing or repeated magma release adds to the
height of underwater volcanoes until they sometimes break the ocean surface as
volcanic islands.You may not have known
this, but the tallest mountain in the world from base to peak is not Mt.
Everest, it’s Moana Kea on the big island of Hawaii.It is 10,200 m tall from base to peak; the
peak of Mt. Everest is only 8,848 m above sea level.

By the way, the deepest part of the
deepest trench in the ocean, the Challenger Deep of the Mariana Trench is
10,916 m deep.That is so deep that if
you put Mt. Everest (8,848 m tall) in it, its peak would still be over 2 km
below the ocean surface!

Figure 2-5. Subduction
of an oceanic plate and formation of an island arc volcano. (Image: ARH)

Volcanic
islands can also occur where a tectonic plate slides over a hot spot in the
mantle where a plume of mantle material pushes through the crust toward the
seafloor. This is how the Hawaiian
Island chain was formed (Fig. 2-7).By
the way, mantle plumes/hot spots can occur on land.A mantle plume is what fuels the geysers and
thermal activity in Yellowstone National Park.

When a
volcanic island does not reach the surface it is a seamount.Actually, a
seamount is any underwater rise that does not reach the surface.Scientists and fishermen discovered that
seamounts are often islands of high biomass and biodiversity surrounded by low
biomass habitats.

The last topographic feature
addressed in this section is the mid-oceanic
ridge.The mid-oceanic ridge is an
undersea mountain range that exists along divergent boundaries where seafloor
spreading occurs (Fig. 2-8).The peak of
mid-oceanic ridges usually rises a few thousand meters above the abyssal plain
on either side of it. The interconnected
mid-oceanic ridge system constitutes the longest continuous mountain range on
the planet.

Zonation of the Ocean

The ocean is divided into the
benthic and pelagic zones (Fig. 2-9).The benthic zone includes the seabed, and the pelagic zone includes the
water column.

Divisions of the
Benthic Zone

The marine
benthos extends from the high tide mark of the intertidal or littoral zone to
the bottom of the deepest trench.The littoral zone benthos includes all
seafloor that is covered and uncovered periodically by tidal exchange.Littoral benthic habitats include rock to mud
substrates.The sublittoral zone extends from the bottom of the littoral zone to
the continental shelf break.Depending
on water turbidity, latitude, and depth, light may reach the seafloor up to a
few hundred meters deep.This is where
we find marine benthic communities including kelp forests, kelp beds, seagrass
beds, turtle grass beds, and coral reefs.The benthic zone of continental slopes is called the bathybenthic or bathyal zone.The abyssal benthic zone includes depths of
the continental rise and abyssal plains.This is the largest benthic habitat in the ocean.The hadal
benthic zone is found only in trenches.The deeper benthic zone is, the less we know about it.

Figure 2-8. Age
of oceanic crust.The newest crust
exists at divergent boundaries at mid-oceanic ridges, and the oldest material
is at trenches. (Image: NOAA)

Divisions of the Oceanic
Zone

The term water column refers to all water from the surface all the way to
the bottom in a particular location.The
water column of the ocean can therefore extend to more than 10,000 m in some
places.Scientists have divided the
water column into divisions by depth and other factors to reduce confusion when
referring to different regions of the ocean.Keep in mind that there is no rigid line separating one division from
the next, and these divisions are used only as general guidelines in discussing
environments at different depths.

All water that is not part of the
intertidal or littoral zone is called the oceanic
zone.The oceanic zone is divided
into water that lies over continental shelves and deeper water.The division over the shelf is called the neritic zone.The rest of the ocean is called the pelagic zone.

The divisions of the open ocean,
starting at the shoreline and moving offshore are indicated in Figure 2-9.The uppermost horizontal layer of the oceanic
zone is the epipelagic zone.It usually extends to a depth of a few
hundred meters.This is also called the photic zone.This zone’s maximum depth is usually defined
as the depth where only 1% of surface incident solar radiation remains.Most pelagic ocean life lives in the
epipelagic zone because this is where photosynthesis can take place.

Figure 2-9.
Divisions of the ocean. (Image: Wikimedia Commons)

The mesopelagic zone spans extends from the bottom of the epipelagic
zone to 700-1000 m. Most solar radiation is absorbed or scattered in the
epipelagic zone, but the mesopelagic zone is not entirely dark.When you are in this zone and you look toward
the surface on a sunny day you can still discern a faint glow.There is too little light here for
phytoplankton to carry out enough photosynthesis to meet their basic energy needs.Because of this, the mesopelagic zone is also
called the disphotic zone.Many animals that live here bio luminesce and
migrate vertically into the epipelagic zone each night in order to feed.

The next layer in the water column
is the bathypelagic zone. The bathylpelagic
zone extends from about 1000 m to depths as deep as 4000 m.The upper bound of this layer is defined as
the depth where surface light is no longer discernible.The lower bound of this layer generally
corresponds with the lower end of the continental slope.Some bathypelagic animals may migrate up into
the mesopelagic zone to feed, but most organisms in the bathypelagic zone feed on
material that drifts down from above as well as on each other.

The abyssalpelagic zone exists below the bathypelagic zone.This layer extends from about 4000m to the abyssal
plain, usually 4000-6000 m deep.Organisms
in the bathypelagic zone make a living by consuming whatever drifts down from
above, by eating each other, and by feeding on benthic organisms.

The deepest pelagic layer is hadalpelagic zone.This zone exists only in trenches, in water
as much as 10,000 m deep.We know the
least about life in the hadalpelagic zone of any ocean depth, though fish were
observed in the Japan Trench via ROV in 2010 at depths approaching 8000 m.

Every division of the ocean has its
own set of challenges and opportunities for organisms that live there.One of the goals of marine biology is to
identify what those challenges and opportunities are, and then discover how
marine organisms exploit them.

Monday, October 1, 2012

Hi -- I am developing a set of readings for my marine biology class. The class is designed for upper division biology majors who have already had a year of introductory biology, including an introduction to ecology.

This is the first reading I have finished (including images, photos, etc.). If you have comments or spot typos, etc., I'd appreciate it if you'd let me know by leaving a comment below.

This reading introduces students to the chemical and physical properties of water - something you MUST understand in order to get a grip on life in water.

**************************************************************

Reading 1: Water

Learning Objectives:

1.Be able to describe the molecular characteristics of water

2.Be able to explain what makes water a polar
molecule

3.Be able to comment on the significance of the
polar nature of water

4.Be able to list and describe physical characteristics
of water

5.Be able to describe the density anomaly of water

Introduction

You have to learn about water if
you want to be a marine biologist.Without
being dramatic, it is safe to say that water is the most important and
biologically valuable substance on Earth.Water is the stuff that makes aquatic environments different than
terrestrial habitats, and most people know amazingly little about water and typically
take it completely for granted until they don’t have it.For one thing, water is the only substance on
Earth that exists naturally and simultaneously in its solid, liquid, and
gaseous states.It’s unique physical and
chemical properties make it a vital substance at all levels of biological organization,
from molecules to the global ecosystem.

Characteristics of Water

Water: A Polar Molecule

Just about everyone knows that H20
is the chemical formula for water.Most people, however, don’t know what water
is like at the molecular level and why this matters. Elemental oxygen has an atomic number of 8.Two of its electrons are in an inner electron
orbital, but the other six electrons are in outer orbitals.A Lewis dot diagram for oxygen shows two
orbitals populated with two electrons and two orbitals containing a single
electron each (Fig. 1-1).The orbitals
with one electron need one more electron each in order to be full and
stable.

Two hydrogen atoms form covalent
bonds with one atom of oxygen to form water.The protons in the hydrogen atoms of H2O repel each other,
and you might assume and that water forms a linear molecule with oxygen in the
middle and hydrogen atoms sticking out both ends, but water is not linear. The hydrogen atoms are actually on the same
side of the oxygen atom.Here’s
why.There are pairs of electrons in oxygen’s
other two orbitals.These are called lone pair electrons.The lone pair electrons are slightly more
repulsive to each other than the protons of the hydrogen atoms are to each
other.This causes the hydrogen atoms to
be displaced to one side of the oxygen atom, and the two lone pair electrons remain
on the other side (Fig 1-2). Because two
of oxygen’s electron orbitals are full it forms bonds with up to only two other
atoms, but if you include the lone pair electrons, water has a tetrahedral
form.The bond angles in a uniform
tetrahedral are all 109.5o, but water is an unequal
tetrahedral.The bond angle between the
hydrogen atoms in water is only 104.45o.This is because the lone pair electrons repel
each other more strongly than the hydrogen atoms do, forcing the hydrogen
nuclei closer together than they would otherwise be.

The unequal distribution of
electrons in water produces partial positive and partial negative charges (Fig.
1-2).A partial positive charge is
associated with each hydrogen nucleus, and a partial negative charge is associated
with each of the lone pair electrons.The
existence of partial charges around a molecule produces a dipole moment.In water this
imbalance is constant so each water molecule experiences a permanent dipole moment.

The permanent dipole moment makes
water a polar molecule.One of the most significant things a polar
molecule can do is form a hydrogen bond with
another polar molecule.A hydrogen bond
forms when a partial positive charge
on one molecule comes into close proximity to a partial negative charge on another polar molecule.These opposite partial electrical charges attract
each other and form a hydrogen bond.Because
hydrogen bonds are weak, they readily form, break, and reform.A water molecule can form up to four hydrogen
bonds at a time (Fig. 1-3).The number
of hydrogen bonds existing between water molecules and between water molecules
and other things is a function of temperature.The number and duration of hydrogen bonds between water molecules determines
whether water is in its solid, liquid, or gaseous form.Water molecules are constantly reoriented in
relation to each other as hydrogen bonds break and reform.Hydrogen bonds form and break 1011
to 1012 times per second in liquid water at 0oC, and 105
to 106 times per second in ice at 0oC.

Water forms hydrogen bonds with
itself and with any other polar substance.This characteristic makes water makes a universal solvent.This
means that any charged atom or polar molecule can dissolve in water.Substances that can dissolve in water include
sugars, salts, ions, etc.Molecules
lacking partial charges, also known as non-polar molecules, cannot readily
dissolve in water.The ability of water
to form hydrogen bonds with other polar molecules gives it an adhesive nature.A meniscus along the inner wall of a glass
tube is an evidence of this characteristic (Fig. 1-6).The adhesive nature of water combined with
its cohesiveness also produces capillary action.

Figure 1-4. The
adhesive nature of water produces a meniscus because water adheres to the inner
wall of the glass cylinder. (Image: ARH)

Water is also cohesive.This characteristic
allows capillary action to occur, and it also produces the phenomenon called surface tension.A water molecule in the middle of a mass of
water can form bonds in any direction, but water molecules at the surface can
form bonds only laterally along the surface and with molecules below them (Fig.
1-5).Remember that water is
tetrahedral.When molecules at the
surface bond with their neighboring water molecules the natural tendency of
these bonds is to be bent, but bonds between surface water molecules are put
under stress since they also bond to molecules below them.The molecules below the surface pull down on
the water molecules above them. This downward
stress makes bonds between surface water molecules flatter than normal.This is what produces surface tension, and surface
tension exists in bodies of water ranging from small droplets in the air to the
entire ocean.

Figure 1-5.
Bonding directions of water at the surface and in the middle of a body of
water.The darker upper layer is where horizontal
hydrogen bonds are under physical stress and create surface tension. (Image:
ARH)

Surface tension can be a significant
factor in aquatic systems.Plankton,
organic matter, and other substances can be trapped in or on it.In the lab even dense objects can be
supported by surface tension (Fig. 1-6).Surface tension increases in cold water and when water contains
dissolved salts.Surface tension is
weaker in warm water and when dissolved organics, fats, oils, and even floating
plants are present.A few organisms, such
as water striders, live most of their life on the surface tension of water.The thin microhabitat in and on the surface
tension is called the neuston.

Specific heat is the amount of energy in calories needed to raise
the temperature of 1.0 g of a substance 1oC.Water has an extremely high specific heat of
1.0, the highest of any common substance on Earth.This means that water has to absorb a lot of
energy to increase its temperature even a little bit.This also means that water must lose a lot of
energy before it cools even a little bit.This tendency to be able to gain or lose large amounts of energy without
much heat flux is called thermal inertia,
and makes water a vital component of living things.For example, the most abundant substance in
our bodies is water, so our bodies resist thermal fluctuations and increases our
ability to maintain a constant internal core temperature even when atmospheric
temperature fluctuates widely.What this
means for aquatic organisms is that since they live in water their environmental
temperature is extremely stable compared to that of air.This also means that the ocean can take up a
massive amount of energy in the tropics and be moved in surface currents
hundreds to thousands of miles to higher latitudes, thus moderating the Earth’s
climate at low and high latitudes.

In addition, water exhibits a
latent heat of melting, fusion, evaporation, and sublimation.The latent
heat of melting is 79.72 cal g-1 of ice at standard temperature
and pressure (STP).This means that in
order for ice at 0oC to melt into liquid water at 0oC, each
gram of ice has to take up 79.72 cal of energy.The latent heat of fusion, the
reverse process to melting, means that to change liquid water at 0oC
into ice at 0oC each gram of water has to lose 79.72 cal.The latent
heat of evaporation is much higher, 540 cal g-1.This is the amount of energy that must be absorbed
by liquid water at STP to convert it into water vapor.Of course 540 cal is also released when each
gram of water condenses.Lastly, the latent heat of sublimation is 679 cal g-1
at STP.This is the amount of energy
each gram of ice has to absorb in order to release molecules of water directly
as water vapor.The latent heat of
melting, fusion, evaporation, and sublimation show us that water has to gain or
release large amounts of energy in order to undergo phase changes.

Density Anomaly of Water

Density is a function of an
object’s mass divided by its volume.Pure
water has a density of 999.84 kg m-3 at STP and maximum density of 1000
kg m-3 at 3.98oC.By comparison, air has a density of 1.29 kg m-3 at STP, so water
is about 775 times denser than air.As a
result, water provides objects far greater buoyancy than air. Density is therefore a significant factor for
aquatic organisms. Water density is affected by temperature and salinity.

Temperature affects water density
as indicated in Fig. 1-7.The density of
pure liquid water increases from about 958 kg m-3 at 100oC
to a maximum density of 1000 kg m-3 at 3.98oC. Its density then declines slightly until it
reaches 0oC. When water loses
enough additional heat to reach the latent heat of fusion it undergoes a phase
change into its crystalline form – ice.When this happens its density drops
precipitously to 917 kg m-3.When water is a liquid, even at 0oC, it forms and breaks
hydrogen bonds so rapidly that one water molecule may be bonded to one, two,
three, or even four other water molecules at a time, but the duration of those
bonds is so fleeting that a crystalline structure is not produced.When water freezes, however, enough energy is
lost from the water that when bonds are formed between water molecules those
bonds are retained long enough to form a 3D crystalline structure.The crystalline structure of ice is made up
of interconnected rings of six water molecules each (Fig. 1-8).The tetrahedral bonding between water
molecules in ice keeps those molecules farther away from each other on average
than they are in the liquid water.The
resulting 3D lattice crystalline structure of ice includes spaces within and
between 6-member rings.These spaces make
ice less dense than water.The density
difference between liquid water and ice is called the density anomaly of water.This explains why ice floats in water.

Figure 1-8.
Three-dimensional structure of ice. Spaces within the 6-member rings and between
6-member rings make ice less dense than liquid water.Blue balls are water molecules.Black dashed lines are hydrogen bonds between
water molecules in the foreground.Gray
dashed lines are hydrogen bonds between water molecules in the background.Red dashed lines are hydrogen bonds between
water molecules in the foreground and the background. (Image: ARH)

Water is the only commonly
occurring substance on Earth that is less dense as a solid than as a liquid.This single characteristic of water allows
life, as we know it to exist on Earth.If
water were like just about everything else, denser as a solid than a liquid, ice
would sink to the bottom of lakes and the ocean whenever it forms.The ocean would become super-cooled and then
filled with ice, and it would never melt.Eventually all bodies of water of Earth would become solid masses of ice
except perhaps for a thin layer of water at the surface melted by solar
radiation.But because ice floats, ice actually
forms an insulating layer at the top of bodies of water, allowing water
underneath to remain liquid even when the air above ice reaches temperatures
far below freezing, allowing organisms below the ice to survive.

The density of water is also affected
by salinity.The temperature-density
curve for water is non-linear (Fig. 1-7).This is not the case for salinity (Fig. 1-9).Temperature alone can increase water density
to 1000 kg m-3, but by adding salt or other solutes, water density
be increased well above that.Water can
dissolve up to about 38 g of salt per 100 g of water at STP, but seawater
normally contains only about 3.3-3.7 g of salt per 100g of water.

Figure 1-9. The
salinity - density relationship for water at 15oC.(Image: ARH)

The importance of water density
cannot be overstated, since density differences are what keep masses of water of
different densities from mixing with each other.Figure 1-10 shows the relationship between
temperature differences and the relative thermal resistance to mixing resulting
from those temperature-density differences.These data show that a difference of even a few degrees is enough to
provide a significant resistance to mixing, as evidenced by the large relative
thermal resistance that exists in the thermocline shown in Fig. 1-10.These density differences are enough to keep
warm surface waters of the epipelagic zone from mixing with deeper, colder
water, and are enough to keep warmer inshore waters from mixing with colder
offshore waters.At the same time,
density driven by temperature and salinity drive the world’s most important
deep ocean current – the global conveyor belt.This current plays a major role in regulating Earth’s climate.

Figure 1-10. Typical mid-summer temperature profile and relative thermal resistance values in a small
temperate lake. Relative thermal
resistance indicates the resistance of successive layers of water to mix with
each other based on resistance of freshwater at 4oC to mix with
freshwater at 5oC. (Image: ARH)

Conclusion

The chemical and physical
characteristics of water determine how it bonds, what dissolves in it, how
dense it is, and how resistant neighboring water masses are to mix with each
other.The resistance to mixing between
successive layers of water limits the movement of nutrients, dissolved oxygen,
plankton, and other things suspended or dissolved in the water.

In summary, by gaining a better
understanding of the molecular and chemical nature of water, you can better
imagine what life is like for aquatic creatures.You can also gain a greater appreciation for
how the ocean helps moderate and maintain living conditions on Earth.

Review Questions

1)Explain what makes water a tetrahedral molecule.

2)Explain why water is a polar molecule and why
this matters.

3)Explain the density anomaly of water and why
this matters.

4)Define the term “relative thermal resistance”
and how this limits water mixing, and then comment on why this matters.